1. Field of the Invention
The present invention relates generally to improved methods for determining production characteristics of a subterranean formation, and more specifically relates to improved methods for determining the production rate of liquid recovery from a subterranean formation during a closed-chamber drill stem test.
2. Description of the Related Art
A drill stem test is a temporary completion of a particular strata or formation interval within a well. It is common in the industry to perform drill stem tests in order to determine useful information about the production characteristics of a particular formation interval.
In a conventional drill stem test, various tools are run into the well on a drill string. The number and types of tools available for use during a drill stem test are many and varied. However, in reality, only five tools are necessary to accomplish a drill stem test: drill pipe, a packer, a test valve, a perforated pipe, and instrumentation for measuring various properties of the well.
The drill pipe carries the tools to the bottom of the well and acts as a conduit into which well fluid may flow during the test. The packer seals off the reservoir or formation interval from the rest of the well and supports the drilling mud (if present) within the annulus during the test. The test valve assembly controls the test. It allows the reservoir or formation interval to flow or to be shut-in as desired. The perforated pipe, generally located below the packer, allows well fluid to enter the drill pipe in an open hole drill stem test. If the drill stem test is of a cased hole, the casing itself will have perforations. The instrumentation, typically pressure and temperature gauges, transduce properties of the well as a function of time.
Conventional drill stem tests consist of recording data from the well as the test valve is opened and well fluid is allowed to flow toward the surface. The time during which the test valve is open and the well is allowed to flow is called a "flow period." The resulting pressure and temperature data are then used to predict production capabilities of the tested formation interval in a manner well known in the art. In a conventional open flow drill stem test, the well fluid is allowed to flow to the surface (if possible) and typically on toward a pit. In a conventional closed chamber drill stem test, the well fluid is not allowed to flow to the surface but is allowed to flow into a closed chamber typically formed by the drill pipe.
Conventional drill stem tests are capable of determining the productivity, permeability-thickness, pressure, and wellbore damage of the tested formation interval as is well known in the art. The productivity, or the well's ability to produce fluid, is determined from the flow and shut-in periods. The productivity of the interval, used in combination with the rate of pressure recharge during periods when the interval is shut-in (i.e, the test valve is closed) yields an idea of the interval permeability-thickness. If interval pressure builds to near stabilization during the shut-in periods, interval pressure may be estimated. Finally, a comparison of flow and shut-in data yields an estimate of wellbore damage.
The quality of the formation characteristics determined from a conventional drill stem test are highly dependent upon the quality of the measurement of dynamic pressure. The ability of a pressure transducer to accurately measure small dynamic pressure changes greatly affects the results of conventional drill stem test data.
For high permeability-thickness wells, sensitive pressure transducers are required. High permeability-thickness wells are prone to rapid pressure changes. Thus, to measure the pressure changes as a function of time, the pressure measurements have to be made quickly and accurately. Pressure transducers that have high sensitivity can also measure and record pressures at higher frequencies. Moreover, in highly permeable wells the draw-down pressure may only be a few psi. To accurately measure this dynamic pressure trend, the gage sensitivity has to be significantly less than the draw-down pressure.
In a conventional closed chamber drill stem test, the influx of well fluids into the closed chamber causes the chamber pressure at the surface to increase. This increase in pressure over time is used to approximate the volume of well fluids produced by standard pressure-volume-temperature relationships well known in the art. L. G. Alexander of Canada was perhaps the first to introduce this method of approximating the volume of well fluids produced during a closed chamber drill stem test.
One of the problems inherent in this technique is that the well fluids produced are typically multi-phase in character (e.g., gas and liquid). During the test, the surface pressure is used to determine the volume of liquid produced or the volume of gas produced depending upon which phase predominates. Unfortunately, even the presence of small amounts of gaseous well fluid can create a large difference in the calculated amount of well fluids produced based on an all-liquid well fluid analysis.
Once the closed chamber test is completed, the amount of liquid well fluid produced can be measured. Down hole pressure gauge measurements can be used with the amount of liquid production to determine the liquid production history during the drill stem test. With the production of liquid well fluids known for a given interval of time during the test, it can be determined whether the liquid production alone was sufficient to produce the surface pressure measurements recorded during that interval. If the liquid production alone cannot account for the surface pressure changes, a multi-phase pressure-volume-temperature relationship can be used to approximate the incremental gas fluid production that would account for the surface pressure change. A fairly accurate (but non-real time) production history can be obtained in this manner for the further determination of reservoir properties.
Thus, conventional drill stem tests, whether open flow or closed chamber, suffer from various errors and uncertainties inherent in measuring and recording dynamic pressure during the flow periods and shut-in periods, and from multi-phase well fluids which hamper the real time determination of well fluid production.
The present invention is directed to an improved method of determining formation interval parameters during a drill stem test by utilizing an acoustic sounding device to accurately determine liquid well fluid level. Accordingly, the present invention provides a new method for more accurately determining the volume of liquid recovery during drill stem testing.